Endogenous opioid peptides are involved in the mediation or modulation of a variety of mammalian physiological processes, many of which are mimicked by opiates or other non-endogenous opioid ligands. Some of the effects that have been investigated are analgesia, tolerance and dependence, appetite, renal function, gastrointestinal motility, gastric secretion, learning and memory, mental illness, epileptic seizures and other neurological disorders, cardiovascular responses, and respiratory depression.
The fact that the effects of endogenous and exogenous opioids are mediated by at least three different types [mu (xcexc), delta (xcex4), kappa (xcexa)] of opioid receptors raises the possibility that highly selective exogenous opioid agonist or antagonist ligands might have therapeutic applications. See W. R. Martin, Pharmacol. Rev., 35, 283 (1983). Thus, if a ligand acts at a single opioid receptor type or subtype, the potential side effects mediated through other opioid receptor types can be minimized or eliminated.
The prototypical opioid antagonists, naloxone and naltrexone, are used primarily as pharmacologic research tools and for the reversal of toxic effects of opioids in case of overdose. Since these antagonists act at multiple opioid receptors, their applications in other therapeutic areas or as pharmacologic tools appear to be limited. However, naltrexone recently was reported to reduce the incidence of relapse in recovering alcoholics by J. R. Volpicelli et al., Opioids, Bulimia and Alcohol Abuse and Alcoholism, L. D. Reid, ed., Springer-Verlag (1990) at pages 195-214. Naloxone has been reported to suppress ethanol but not water intake in a rat model of alcoholism. J. C. Froehlich et al., Pharm. Biochem. Behav., 35, 385 (1990).
Some progress has been made in the development of highly selective opioid antagonists. For example, Portoghese et al. (U.S. Pat. No. 4,816,586) disclose certain opiate analogs which possess high selectivity and potency at delta receptors. Minimal involvement was observed at mu and kappa opioid receptors. One of the highly selective analogs disclosed in U.S. Pat. No. 4,816,586 has been named xe2x80x9cnaltrindolexe2x80x9d or xe2x80x9cNTI,xe2x80x9d and has the formula: 
wherein X is NH. See P. S. Portoghese et al., J. Med. Chem., 31, 281 (1988).
Portoghese et al. (U.S. Pat. No. 4,649,200) disclose substituted pyrroles which exhibit selective antagonism at kappa opioid receptors. One such analog is norbinaltrophimine (norBNI), which has the formula: 
The selectivities of these prototypical xcex4 and xcexa opioid receptor antagonists have been attributed to the presence of nonpeptide xe2x80x9caddressxe2x80x9d mimics which bear a functional relationship to key elements in the putative xcex4 and xcexa addresses of enkephalin and dynorphin, respectively. See, P. S. Portoghese et al., Trends Pharmacol. Sci., 10, 230 (1989). Accordingly, the design of NTI employed a model that envisaged the Phe4 phenyl group of enkephalin as a critical part of the xcex4 address. See, P. S. Portoghese et al., J. Med. Chem., 33, 1714 (1990). Similarly, the address element conferring selectivity in norBNI has been suggested to be a basic function that mimics the guanidinium moiety of Arg7 in dymorphin, by P. S. Portoghese et al., J. Med. Chem., 31, 1344 (1988).
It has recently been reported that suppression of ethanol ingestion may be mediated by the delta opioid receptor type. For example, the xcex4 antagonist, N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174864), strongly inhibits ethanol drinking, but has a very short duration of action, which may limit its clinical utility. See J. C. Froehlich et al., Psychopharmacol., 103, 467 (1991). Using NTI as an antagonist, M. Sofuoglu et al., J. Pharmacol. Exp. Ther., 257, 676 (1991) determined that the antinociceptive activity of two delta receptor agonist enkephalin analogs, DSLET and DPDPE, may be mediated by two discrete delta opioid receptor subtypes. It has also been suggested that development of addiction and/or tolerance to opiates may be inhibited by delta-opioid receptor antagonists, and that opioid-type delta-opioid receptor antagonists may be useful as immunosuppressive agents. Likewise, compounds which are selective at mu receptors may be useful as analgesics which do not exhibit the potentially harmful side effects of less-selective analgesics such as morphine.
Therefore, a continuing need exists for compounds which are opioid receptor-selective, i.e., which can act as agonists or antagonists with specificity at the delta, mu or kappa opioid receptor, or at one of the subtypes of these receptors.
The present invention is directed to biologically active compounds of formula (I): 
wherein R1 is (C1-C5)alkyl, C3-C6(cycloalkyl)alkyl, C5-C7-(cycloalkenyl)alkyl, (C6-C12)aryl, (C6-C12)aralkyl, trans-(C4-C5)alkenyl, allyl or furan-2-ylalkyl, R2 is H, OH or O2C(C1-C5)alkyl; R3 is H, (C6-C10)aralkyl, (C1-C5)alkyl or (C1-C5)alkylCO; X is O, S or NY, wherein Y is H or (C1-C5)-alkyl; R4 is CH2 (methylene) or Cxe2x95x90O (carbonyl), R5 is CH2, Cxe2x95x90O or Cxe2x95x90NH (imino) and R6 is (C1-C4)alkyl or NH(C1-C4)alkyl, optionally substituted by a non-terminal (C1-C2)alkyl group or by N(R7)(R8) wherein R7 and R8 are individually H or (C1-C3)alkyl, with the proviso that one of R4 or R5 is CH2, and the pharmaceutically acceptable salts thereof.
Using peptide antagonists of known binding selectivity as standards, it was unexpectedly found that the compounds of the invention are selective antagonists at kappa opioid receptors, while exhibiting little or no binding at delta or mu receptors. Thus, the present invention also provides a method for blocking kappa opioid receptors in mammalian tissue comprising contacting said receptors in vivo or in vitro with an effective amount of the compound of formula I, preferably in combination with a pharmaceutically acceptable vehicle. Thus, the compounds of formula I can be used as pharmacological and biochemical probes of opiate receptor structure and function, e.g., to measure the selectivity of other known or suspected opioid receptor antagonists or agonists. Such tissue includes tissue of the central nervous system (CNS), the gut, the cardiovascular system, the lung, the kidney, reproductive tract tissue and the like. Therefore, the compounds of formula I which exhibit kappa receptor antagonist activity may also be therapeutically useful in conditions where selective blockage of kappa receptors is desired. This includes blockage of the appetite response, blockage of paralysis due to spinal trauma and a variety of other physiological activities that may be mediated through kappa receptors.
The alkyl moiety present in the R1 group which links the cycloalkyl, cycloalkenyl, aryl, or furan-2-yl moiety to the basic nitrogen atom in the compounds of formula I is a lower(alkyl) group, preferably xe2x80x94(CH2)nxe2x80x94, wherein n is about 1-5, most preferably n is 1, e.g., R1 is C3-C6(cycloalkyl)methyl, C5-C7(cycloalkenyl)-methyl, arylmethyl or furan-2-yl-methyl. Preferred aryl moieties include (C6-C10)aryl, i.e., phenyl, benzyl, tolyl, napthyl, xylyl, anisyl and the like.
In structure I, a bond designated by a wedged or darkened line indicates one extending above the plane of the R3O-substituted phenyl ring. A bond designated by a broken line indicates one extending below the plane of the phenyl ring.
Preferred compounds of the formula I are those wherein R1 is (C1-C5)alkyl, C3-C6(cycloalkyl)alkyl or C5-C7-(cycloalkenyl)alkyl, preferably wherein R1 is C3-C6(cycloalkyl)methyl, and most preferably wherein R1 is cyclopropylmethyl. R2 is preferably OH or OAc (O2CCH3), and R3 preferably is H. Preferably, X is NH or NCH3, most preferably NH. Preferably, R6 is methyl, ethyl, propyl, butyl or 2-methylbutyl which is unsubstituted or is terminally substituted with N(CH3)2 or N(CH2CH3)2. Preferably, R4 is CH2 and R5 is Cxe2x95x90O or Cxe2x95x90NH.
Since the compounds of the invention are formally morphinan derivatives, it is believed that their ability to cross the xe2x80x9cblood-brain barrierxe2x80x9d and to affect the central nervous system (CNS) should be far superior to peptide opioid antagonists. For example, as disclosed in U.S. patent application Ser. No. 07/750,109, filed Aug. 26, 1991, both NTI and its benzofuran analog, NTB were found to produce unexpectedly prolonged suppression of ethanol drinking in rats that were selectively bred for high voluntary ethanol drinking, as compared to peptidyl delta-opioid receptor antagonists. Processes of preparing the compounds of formula I are also aspects of the invention, as described hereinbelow, as are the novel intermediates employed in the syntheses.
The compounds of formula I wherein X is NH can be readily synthesized by reaction of a 4,5-epoxy-6-keto-morphinan such as naltrexone (6) with 4-hydrazinobenzonitrile (D. E. Rivett et al., Austr. J. Chem., 32 1601 (1979)) under Fischer indole conditions, as shown in Scheme I, to yield the 5xe2x80x2-nitrile 7. 
Nitrile 7 was reduced to primary amine 8 using Raney Ni, and 8 was reacted with the appropriate imidate esters, of the formula CH3OC(xe2x95x90NH)xe2x80x94R, to yield amidates of general formula II, wherein R is ethyl, 2-methylbutyl, methyl, pentyl, propyl and butyl, respectively, for compounds 1-6.
See, S. R. Sandler et al., Organic Chemistry, Vol. 3, Academic Press, NY (1972) at pages 268-299; and E. Cereda et al., J. Med. Chem., 33, 2108 (1990).
Compounds of formula I wherein R4 is Cxe2x95x90O can be prepared by reacting a morphinan such as naltrexone with 4-carboxyphenylhydrazine to yield a compound of formula 7 wherein the 5xe2x80x2-CN group has been replaced by a 5xe2x80x2-carboxy group, i.e., compound 15, hereinbelow. The 5xe2x80x2-carboxy intermediate is then amidated, e.g., with an alkylamine of the formula H2NR5R6, wherein R5 is CH2 and R6 is as defined above, to yield the final products.
Compounds of formula I wherein R4 is CH2 and R5 is Cxe2x95x90O can be prepared by reacting the 5xe2x80x2-CH2NH2 group, i.e., of compound 8 with a carboxylic acid of the general formula HO2CR6 wherein R6 is as defined above, in the presence of benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (xe2x80x9cBOP Reagent,xe2x80x9d Aldrich Chem. Co.).
Compounds of formula I wherein R4 is CH2, R5 is Cxe2x95x90NH and R6 is NH(C1-C4)alkyl, optionally substituted by non-terminal (C-C2)alkyl or by N(R7)(R8) can be prepared by reacting, i.e., a compound of formula 8 with a compound of the formula R6xe2x80x94C(OMe)xe2x95x90NH, wherein R6 is as defined immediately above.
Compounds of formula I wherein X is O, S or NY can be prepared from intermediates analogous to 7 or 8 wherein NH has been replaced by O, S or NY. These intermediates can be prepared as generally disclosed in U.S. Pat. No. 4,816,586, which is incorporated by reference herein, which also discloses methods suitable for the preparation of salts of compounds of formula I.
The structures, common names and Merck Index reference numbers of representative 4,5-epoxy-6-ketomorphinan starting materials of general formula (III) are summarized on Table I, below.
Other starting materials of the general formula III can be prepared by synthetic methods which are well known in the art of organic chemistry. For example, compounds wherein R1 is H and R3 is a suitable protecting group, and wherein the 6-keto group has also been protected, can be prepared from the compounds of formula III. These intermediates can be N-alkylated and deprotected to yield compounds of formula I wherein R1 is C2-C5(alkyl), C4-C6(cycloalkyl)alkyl, C5-C7(cycloalkenyl)alkyl, aryl, aralkyl, trans-C4-C5alkenyl or furan-2-ylalkyl, by the application of well-known reactions.
For example, the free hydroxyl groups of the compounds of formula III, e.g., R2=OH and/or R3=H, can be protected by acid-labile groups such as tetrahydropyranlyl, trimethylsilyl, 1-methoxy-isopropyl and the like as disclosed in Compendium of Organic Synthetic Methods, I. T. Harrison et al., eds., Wiley-Interscience, New York, N.Y. (1971) at pages 124-131, (hereinafter xe2x80x9cCompendiumxe2x80x9d). The protection of the 6-keto group of compounds of Table I by its reversible conversion into a ketal or a thioketal group is disclosed in Compendium, at pages 449-453. Methods for the demethylation of N-methyl amines have been disclosed, for example, in Compendium at page 247, J. Amer. Chem. Soc., 89, 1942 (1967) and J. Amer. Chem. Soc., 77, 4079 (1955).
Procedures for the alkylation of secondary amines with halides under basic or neutral conditions are well known. For example, see Compendium at pages 242-245; Orq. Synth., 43, 45 (1963); J. Org. Chem., 27, 3639 (1962) and J. Amer. Chem. Soc., 82, 6163 (1960).
Compounds of formula III wherein R2 is acyloxy and/or R3 is acyl can be prepared by using the corresponding starting materials on Table I. For example, naltrexone can be diacylated by reacting it with the appropriate (C1-C5)alkyl anhydride for 10-18 hrs at 18-25xc2x0 C. The resultant 3,14-diacylated compound can be converted to the 14-acylated compound by limited hydrolysis. The 3-acylated starting materials can be prepared by the short-term reaction of the compounds of Table I with the anhydride, e.g., for about 2-4 hours. The 3-acylated product can be separated from the 3,14-diacylated product by chromatography.
The acid salts of compounds of formula I wherein R3=H, can be converted into the corresponding (C1-C5)alkoxy derivatives [R3=(C1-C5)alkyl] by dissolving the starting material in DMF and adding an excess of the appropriate (C1-C5)alkyl iodide and an amine such as diisopropylethylamine. The reaction can be conducted at an elevated temperature for about 4-10 hours. The final product can be purified by column chromatography.
The invention also comprises the pharmaceutically acceptable salts of the biologically active compounds of formula I together with a pharmaceutically acceptable carrier for administration in effective, non-toxic dose form. Pharmaceutically acceptable amine salts may be salts of organic acids, such as acetic, citric, lactic, malic, tartaric, p-toluene sulfonic acid, methane sulfonic acid, and the like as well as salts of pharmaceutically acceptable mineral acids such as phosphoric, hydrochloric or sulfuric acid, and the like. These physiologically acceptable salts are prepared by methods known in the art, e.g., by dissolving the free amine bases with an excess of the acid in aqueous alcohol.
In the clinical practice of the present method, the compounds of the present invention will normally be administered orally or parenterally, as by injection or infusion, in the form of a pharmaceutical unit dosage form comprising the active ingredient in combination with a pharmaceutically acceptable carrier, which may be a solid, semi-solid or liquid diluent or an ingestible capsule or tablet. The compound or its salt may also be used without carrier material. As examples of pharmaceutical carriers may be mentioned tablets, intravenous solutions, suspensions, controlled-release devices, microcapsules, liposomes and the like. Usually, the active substance will comprise between about 0.05 and 99%, or between 0.1 and 95% by weight of the resulting pharmaceutical unit dosage form, for example, between about 0.5 and 20% of preparation intended for injection or infusion and between 0.1 and 50% of preparation, such as tablets or capsules, intended for oral administration.
The invention will be further described by reference to the following detailed examples, wherein melting points were taken using a Thomas-Hoover Melting Point apparatus in open capillary tubes, and are uncorrected. NMR data was collected at ambient temperature on a Bruker AC-200 or a GE Omega 300, using DMSO-d6, and TMS as the internal reference. IR data in each case was obtained from a KBr disk on a Nicollet FT-IR instrument. Low resolution FAB mass data was obtained on a Finnigan 4000 instrument. Ion spray mass spectral data was obtained from the Biochemistry Dept. Chemicals were supplied through Aldrich, except where noted. Naltrexone hydrochloride was obtained from Mallinkrodt, BOP reagent was obtained from Peptides International. TLC Rf values were obtained on silica gel using the following solvent systems: (A) 33% EtOAc, 33% CHCl3, 33% MeOH, 1% NH3; (B) 95% CHCl3, 5% MeOH, 0.1% NH3; (C) 45% EtOAc, 45% MeOH, 5% NH3. Analytical data was supplied through M-H-W Laboratories, Phoenix, Ariz.
Physiological data for guinea pig ileal longitudinal muscle (GPI) was obtained by the methodology of H. P. Rang, xe2x80x9cStimulant Actions of Volatile Anaesthetics on Smooth Muscle,xe2x80x9d Brit. J. Pharmacol., 22, 356 (1964) on Dunkin-Hartley males; mouse vas deferens (MVD) data was obtained using Henderson""s method on Swiss-Webster males (G. Henderson et al., xe2x80x9cA New Example of a Morphine-Sensitive Neuroeffector Junction: Adrenergic Transmission in the Mouse vas Deferens,xe2x80x9d Brit. J. Pharmacol., 46, 764 (1972). Guinea pig brain membrane binding assays were done on Dunkin-Hartley males using a modification of the method of L. L. Werling et al., xe2x80x9cOpioid Binding to Rat and Guinea Pig Neural Membranes in the Presence of Physiological Cations at 37xc2x0 C.,xe2x80x9d J. Pharmacol. Exp. Ther., 233 722 (1985). In vivo assays were done s.c. on mice.